As a steering device for providing a steering angle to steering wheels (generally, front wheels, except for special vehicles such as forklifts), for example, a structure as shown in FIG. 9 is widely known. In this steering device, a steering shaft 3 is rotatably supported on an inner side of a cylindrical steering column 2 which is supported on a vehicle body 1. A steering wheel 4 is fixed to a rear end portion of the steering shaft 3 protruding toward the rear side from a rear end opening of the steering column 2. If the steering wheel 4 is rotated, this rotation is transmitted to an input shaft 8 of a steering gear unit 7 through the steering shaft 3, a universal joint 5a, an intermediate shaft 6 and a universal joint 5b. If the input device 8 rotates, a pair of tie rods 9, 9 provided on both sides of the steering gear unit 7 are pushed and pulled, whereby a steering angle according to the operation amount of the steering wheel 4 is provided to a pair of left and right steering wheels. In this specification and claims, a front-rear direction, a width direction, and a vertical direction refers to a front-rear direction, a width direction, and a vertical direction of a vehicle body on which a telescopic steering device is mounted. That is, in FIG. 9, with respect to the steering column 2, the steering gear unit 7 is positioned on the front side, and the steering wheel 4 is positioned on the rear side.
In this steering device, there has been known a telescopic mechanism for adjusting the position of the steering wheel in the front-rear direction according to the physique or driving position of a driver. In order to configure this telescopic mechanism, a rear portion of an inner column 10 provided on a front side is fitted into a front portion of an outer column 11 provided on a rear side so as to be relatively displaceable in an axial direction, whereby the steering column 2 is configured such that its total length can increase or decrease in a telescopic manner. Also, an outer tube 12 and an inner shaft 13 are assembled by spline engagement or the like, whereby the steering shaft 3 is configured to be able to transmit torque and able to be extended and contracted.
The example shown in FIG. 9 incorporates a tilt mechanism capable of adjusting a vertical position of the steering wheel 4. Further, this example incorporates an electric power steering device which uses an electric motor 14 as an auxiliary power source to reduce a force necessary for operating the steering wheel 4. To this end, on the front end portion of the steering column 2 (the inner column 10), a housing 15 including a worm reducer and the like configuring the electric power steering device is provided, and the housing 15 is supported on the vehicle body 1 to be able to swing around a horizontal shaft 16. On a supporting bracket 17 supported at a different location of the vehicle body 1, a column-side bracket 18 fixed to a portion of the steering column 2 (the outer column 11) is supported to be displaceable in the front-rear direction and the vertical direction.
In the tilt mechanism and the telescopic mechanism except for an electric type, they are required to be switchable between a state where they can adjust the position of the steering column 2 and a state where they can hold the steering column at an adjusted position. An example of a conventional structure of a position adjusting mechanism will be described with reference to FIG. 10 in addition to FIG. 9. In order to make it possible to elastically expand or contract an inner diameter of the front end portion of the outer column 11, a slit 19 is formed on a lower surface of the outer column 11 from the front end portion to a center side portion to be long in the axial direction. A pair of sandwiched portions 20, 20 configuring the column-side bracket 18 are provided on the outer column 11 at positions interposing the slit 19 therebetween in the width direction. At positions of the sandwiched portions 20, 20 which match with each other, front-rear long holes 21, 21 are formed long in the axial direction of the outer column 11. In a pair of supporting plate members 22, 22 which configure the supporting bracket 17 and sandwiches the sandwiched portions 20, 20 from both sides in the width direction and are parallel to each other, vertical long holes 23, 23 are formed to have a partial arc shape about the horizontal shaft 16. Here, the vertical direction of FIG. 10 corresponds to the vertical direction of the vehicle body. An adjustment rod 24 is inserted through the long holes 21, 23.
An expansion/contraction mechanism incorporated in the adjustment rod 24 makes it possible to expand or contract an interval between the supporting plate members 22, 22, thereby making it possible to adjust the position of the outer column 11 in the vertical direction and the front-rear direction and to hold the outer column 11 at the adjusted position. In order to configure the expansion/contraction mechanism, on a portion of a base end portion of the adjustment rod 24 protruding from an outer surface of one supporting plate member 22 (the left one of FIG. 10) of the supporting plate members 22, 22, a base end portion of an adjustment lever 25 is fixed, and a cam device 26 is provided between the outer surface of the one supporting plate member 22 and the adjustment lever 25. The cam device 26 is expanded or contracted in the axial direction based on a relative displacement between a drive-side cam 27 and a driven-side cam 28, and the driven-side cam 28 is engaged with the vertical long hole 23 formed in the one supporting plate member 22 to be displaceable only along the vertical long hole 23 (with rotation thereof being restricted). The drive-side cam 27 is configured to be rotatable together with the adjustment rod 24 by the adjustment lever 25. On a portion of a tip end portion of the adjustment rod 24 protruding from the outer surface of the other supporting plate member 22 (the right one of FIG. 10) of the supporting plate members 22, 22, a nut 29 is fixed, and a thrust bearing 30 and a pressing plate 31 are provided between the other supporting plate member 22 and the nut 29. On an inner surface of the pressing plate 31, an engaging piece 32 is provided, and the engaging piece 32 is engaged with the vertical long hole 23 formed in the other supporting plate member 22 to be displaceable only along that vertically long hole 23.
In order to adjust the position of the steering wheel 4, the adjustment lever 25 is rotated toward a predetermined direction (downward), whereby the drive-side cam 27 is rotated such that the dimension of the cam device 26 in the axial direction decreases. As a result, the interval between the inner surfaces of the driven-side cam 28 and the nut 29 facing each other is expanded, whereby the force of the supporting plate members 22, 22 pressing the sandwiched portions 20, 20 is released. At the same time, the inner diameter of the fitting portion which is the front portion of the outer column 11 and internally fitted with the rear portion of the inner column 10 is elastically expanded, whereby surface pressure acting on the contact area between the inner circumferential surface of the front portion of the outer column 11 and the outer circumferential surface of the rear portion of the inner column 10 is reduced. In this state, the position of the steering wheel 4 in the vertical direction and the front-rear direction can be adjusted within a range in which the adjustment rod 24 can be displaced inside the long holes 21, 23.
After the steering wheel 4 is moved to a desired position, the adjustment lever 25 is rotated toward a direction (upward) opposite to the predetermined direction, whereby the dimension of the cam device 26 in the axial direction is increased. As a result, the interval between the inner surfaces facing each other in the driven-side cam 28 and the nut 29 (which correspond to a pair of pressing members) is contracted, whereby the sandwiched portions 20, 20 are strongly pressed by the supporting plate members 22, 22. At the same time, the inner diameter of the portion of the front portion of the outer column 11 internally fitted with the rear portion of the inner column 10 is elastically contracted, whereby the surface pressure acting on the contact area between the inner circumferential surface of the front portion of the outer column 11 and the outer circumferential surface of the rear portion of the inner column 10 is increased. In this state, the position of the steering wheel 4 in the vertical direction and the front-rear direction is held at the adjusted position.
Incidentally, in order to ensure the flexibility of the portion of the front portion of the outer column 11 fitting on and holding the inner column 10, the front end portion of the slit 19 is opened to a front edge of the outer column 11. In this case, the rigidity of the front edge of the outer column 11 which is the open end side of the slit 19 becomes lowest, and the rigidity of the outer column 11 increases as proceeding toward the bottom end side of the slit 19. Therefore, in accordance with the position of the steering wheel 4 in the front-rear direction, a force necessary for operating the adjustment lever 25 to contract the inner diameter of the front portion of the outer column 11 changes.
In this regard, Patent Document 1 discloses a structure in which a slit includes a main slit portion which is formed in an axial direction and a sub slit portion which is formed in a circumferential direction in at least the front end portion of both end portions of the main slit portion such that an end portion of the main slit portion in the axial direction is open to a center portion of the sub slit portion in the circumferential direction, and the shape of the slit as seen from the radial direction is a T shape or an H shape. With this structure, while it is possible to ensure the flexibility of the fitting and holding portion of the outer column, thereby reducing a force necessary for expanding or contracting the inner diameter of the fitting and holding portion, it is possible to alleviate concentration of stress on the end portions of the slit. However, even in this structure, there is room for improvement in order to stabilize a force necessary for operating the adjustment lever regardless of the position of the steering wheel in the front-rear direction.